This invention relates to an optical disk medium for use in recording and reproducing information or data by the use of a small optical spot and to an optical disk recording/reproducing apparatus.
In order to protect a recording layer having a thickness on the order of microns, data recording/reproducing (read/write) operations for an optical disk is carried out by the use of an optical system. Specifically, a small optical spot is formed by an objective lens through a transparent optical disk substrate having a thickness of 1.2 mm in case of a CD (Compact Disk) and 0.6 mm in case of a DVD (Digital Versatile Disk).
It is known that, in the above-mentioned optical system, relative tilt between the optical disk substrate and the objective lens may cause optical wavefront aberration, resulting in enlargement or asymmetry of the optical spot, which deteriorates recording/reproducing (read/write) characteristics.
An optical disk medium has a disk-like or a circular shape. Therefore, when a center portion of the optical disk medium is placed on a spindle of an optical disk recording/reproducing apparatus, the tilt of the optical disk medium is often caused to occur in a radial direction perpendicular to a recording track formed in a spiral fashion.
In order to suppress the deterioration in recording/reproducing characteristics owing to the above-mentioned tilt, the standard for the optical disk medium limits an allowable tilt upon production of the optical disk medium. On the other hand, the optical disk recording/reproducing apparatus adopts tilt control in which the tilt of the optical disk medium is detected and the tilt of the objective lens is changed and adjusted in correspondence to the tilt of the optical disk medium so as to assure stable recording/reproducing characteristics.
In the tilt control, it is a problem how to detect the tilt of the optical disk medium.
As a known technique, proposal has been made of a method of detecting the tilt by the use of pre-pits in a DVD-RAM (Random Access Memory). Referring to FIG. 1, a pre-pit portion of the DVD-RAM has a track structure which will presently be described. A land recording track 1 and a groove recording track 2 are alternately arranged. On a boundary line between the land recording track 1 and the groove recording track 2, a plurality of first pre-pits 14 and a plurality of second pre-pits 15 are formed. The first and the second pre-pits 14 and 15 are alternately arranged on left and right sides as seen from each of the land and the groove recording tracks 1 and 2.
Upon occurrence of tilt of the optical disk medium in the radial direction perpendicular to the recording tracks, the intensity distribution of the optical spot becomes asymmetrical on the left and the right sides with respect to a recording track center under the influence of aberration. For example, it is assumed that the optical spot follows the groove recording track as depicted by a dash-and-dot line in the figure. Before occurrence of tilt, a first push-pull signal upon passing through each first pre-pit 14 is equal in amplitude to and opposite in sign to a second push-pull signal upon passing through each second pre-pit 15. Once the tilt occurs, the first and the second push-pull signals are different in amplitude because the optical spot is asymmetrical. Therefore, by detecting the signal levels of the push-pull signals when the optical spot passes through the first and the second pre-pits, the tilt can be detected.
However, the change in signal level of the push-pull signal not only occurs by the tilt of the optical disk medium but also occurs when the first and the second pre-pits are not exactly formed at symmetrical positions on left and right sides with respect to the recording track center but are displaced or deviated. In the optical disk medium having a recording track interval on the order of submicron, it is difficult to form the pre-pits at accurate pit positions. Therefore, it is impossible to distinguish whether the change in signal level of the push-pull signal actually results from the tilt of the optical disk medium or from the positioning error of the pre-pits.
In a next-generation high-density optical disk using a blue light source, the allowable tilt is further reduced. Therefore, a method higher in reliability of detection is required.
Japanese Patent Publication (JP-A) No. 2001-118274 discloses a groove comprising wobbled groove parts and straight groove parts alternately connected in order to detect the tilt of the optical disk medium. The wobbled groove part has opposite side walls having a wobbled structure. The straight groove part has opposite side walls having a straight shape without being wobbled.
Japanese Patent Publication (JP-A) No. H10-302319 discloses an optical recording medium in which a land portion or a groove portion has a part whose center line parallel to the tracking direction is deviated from the recording track center.